Reaction sintered silicon nitride has shown considerable promise as a structural ceramic for high temperature applications due to the ease of formation of the product into complex shapes prior to the final reaction sintering step. However, for some applications the final strength of the product is not as high as desired. The current state of the art has been summarized in the recent survey article of Messier and Wong "Ceramics for High Performance Applications," Burke, Gorum, and Katz 2nd Army Material Technology Conference Series (copyright 1974 by Brook Hill Publishing Company). While considerable advances have recently been made, particularly following the teachings of Washburn as described in co-pending application Ser. No. 370,745 filed June 18, 1973, it is desired to obtain even higher strengths than those obtained by Washburn. Certain prior work done by Godfrey and Pitman (see particularly pages 436 and 437 of the same "Ceramics for High Performance Applications" mentioned above) attempted to improve the strength of a reaction sintered silicon nitride body by providing an alumina coating on the body with subsequent heat treatment. However the results, on firing, were found to decrease, rather than increase, the strength of the fired body.
Kirchner in "Chemical Strengthening of Ceramic Materials," (6 December 1971 National Technical Information Service AD735135), makes reference to the treatment of a low strength, reaction sintered silicon nitride body, with boiling tetraethylorthosilicate (see pages 74- 75). The silicate was hydrolyzed by use of HCl and the body was then fired to 975.degree. C. in an unspecified atmosphere. The strength of the body increased from 18,300 p.s.i. to 22,300 p.s.i. No data are given on the use of higher strength starting materials.
While Professor Jack (pages 284 and 285 of the above "Ceramics for High Performance Applications") has made some products of undisclosed "high" strength by reacting mixtures of silicon nitride and alumina to form "beta sialon," no data is given as to what strengths were obtained, with and without the alumina. Also the bodies were not formed by reaction sintering silicon powder; the silicon nitride powder had been prepared separately and was then mixed with the alumina and formed into a shaped body. Jack mentions that, during the firing, there must be a sufficient partial pressure of oxygen in the flowing nitrogen atmosphere to form SiO.sub.2 and thereby prevent volatilization of silicon nitride according to the reaction Si.sub.3 N.sub.4 + 11/2 O.sub.2 .fwdarw.3 SiO + 2N.sub.2. Similarly, the French patent to Mary, Lortholary, Goursat and Billy U.S. Pat. No. 2,221,421 dated Oct. 11, 1974 describes a process for fabricating a body from silicon oxynitride powder which is mixed with alumina, formed into a shaped body, then coated with silicon oxynitride (or a mixture comprising the constituents of silicon oxynitride) and then heated between 1250.degree. and 1850.degree. C. in air, inert gas, or nitrogen. The principal objective of the French patent was to increase the density of the fired product; no data are given on strengths.